The invention relates to a method for checking the tightness of a tank system.
A method for checking the tightness of the tank-venting system of a vehicle is disclosed, for example, in U.S. Pat. Nos. 5,890,474 and 6,131,550.
In the above, an overpressure is introduced into a tank-venting system by means of a pressure source and the time-dependent trace of at least one operating characteristic variable of the pressure source is detected while introducing the overpressure. This time-dependent trace is compared to a time-dependent trace of the operating characteristic variable (diagnostic trace), which is expected in the case of a tight tank system. This expected time-dependent trace is, for example, previously measured, computed or estimated. A conclusion is then drawn as to non-tightness when the detected time-dependent trace deviates from the diagnostic trace by at least a pregiven value. The pump current of a pump is used, for example, as the diagnostic trace. The diagnostic trace corresponds to the pump current which occurs when an overpressure is introduced into a tank-venting system having a pregiven reference leak having, for example, a diameter of 0.5 mm. In this case, the current of the pump is compared to a reference current during introduction of the overpressure. The absolute current level can vary greatly for the same leak size because of ambient conditions and tolerances. Independently of the absolute level, a fine leak having a diameter of 0.5 mm can, however, be excluded if, during the introduction of the overpressure by pumping, a current level is reached which is greater than the reference current. If this current level is not reached, then a conclusion can be drawn as to a fine leak having a diameter of xe2x89xa70.5 mm.
In contrast, a large leak having a diameter of 1 mm or more cannot be detected from the method of tightness checking of tank-venting systems known from the state of the art. When pumping against a reference leak, and when a leak is present having a diameter of approximately 1 mm, any desired current level can be reached after a certain pregivable time in dependence upon ambient conditions and component tolerances. A reliable decision is therefore not possible in this manner as to whether a large is leak is present having a diameter of 1 mm or more.
In view of the above, it is, for example, conceivable that a pregiven pump current is not yet reached after a pregiven time for a leak having a diameter of only 0.4 mm (for which no fault output is required), so that a fault would be outputted in the methods known from the state of the art.
On the other hand, for other tolerances, a pregiven current threshold can be reached also for a leak having a diameter of more than 1 mm in a time interval, which is less than a pregiven diagnostic time interval and this would lead to the situation that a fault output would not take place.
In view of the above, it is a task of the invention to improve a method for checking the tightness of a tank system of a vehicle so that a reliable detection of large leaks having a diameter of xe2x89xa71 mm is possible independently of tolerance-caused disturbances and/or environmentally-caused disturbances of the operating characteristic value during the introduction of an overpressure into the tank system.
The advantage of the method of the invention is that, to a certain extent, it is possible to verify an initially expected untightness or to show that it is false via the following: a further diagnostic trace (that is, a further introduction of an overpressure or underpressure into the tank system); the detection of the operating variable of the pressure source when introducing this additional overpressure or underpressure until reaching a second time point or a second pressure level; a comparison of the so detected operating characteristic variable to a time-dependent trace of the operating characteristic variable (expected for a tight tank system) when introducing the additional overpressure; and, the output of a fault announcement only when a determination is made of a deviation of the detected operating characteristic variable from the diagnostic trace by at least a pregiven value and when there is a deviation of the operating characteristic variable from the further diagnostic trace by at least a pregiven value. With this verification or showing to be false, faults can be eliminated which are caused especially by ambient influences or component tolerances.
With the method of the invention, large leaks having a diameter of 1 mm or greater than 1 mm can be recognized with high accuracy. On the other hand, leaks having a diameter of less than 1 mm do not lead to fault announcements.
The electric current of an overpressure pump/underpressure pump is advantageously detected as an operating characteristic variable of the pressure source. For this reason, it is not only possible to accurately detect significant changes of the operating state of the pressure source but the detected electric current can also be directly further processed in evaluation circuits or the like in an advantageous manner.
As to the time-dependent trace of the operating characteristic variable (that is, the electric current), the current could, in principle, be detected continuously over time and the functional trace shown in this manner could be compared to a time-dependent trace which was previously measured, computed or estimated.
An especially advantageous embodiment provides that the pump current is compared to the expected current values for a tight tank system after the elapse of pregiven time intervals and a fault announcement is only outputted when the detected pump current, after the elapse of the first time interval, deviates from an expected current value for a tight tank system as well as, after the elapse of the second time interval, deviates from a second expected current value for a tight tank system.
With the comparison of these values, a very accurate leak detection of a tank-venting system is also possible. A significantly reduced storage complexity is required when compared to the detection of the time-dependent trace of the current intensity.